1. Field of the Invention
The present invention relates to a distortion-compensation apparatus, and more particularly to an adaptive distortion-compensation apparatus that can be applied to a high-frequency power amplifier.
2. Description of the Related Art
With a recent increase in the speed and capacity of the communication, there has been a more severe demand for linearity that is made of the transmission power amplifier in the digital wireless communication apparatus. This simultaneous has raised the problem of obstructing the increase in the power efficiency of the power amplifier.
On the other hand, the continuous phone talk duration of the digital portable telephone has solely become increasingly longer. For this reason, when putting a new digital wireless communication apparatus into the market, from the viewpoint of the product""s competing ability the increase in the time period of continuous use thereof has been becoming non-ignorable. Nowadays, therefore, the movement of improving the power efficiency by introducing a technique of compensating the distortion of the power amplifier has been becoming more and more vigorous. However, such a distortion-compensation technique has become very large in terms of the scale of its relevant circuit. Therefore, that distortion-compensation technique has become virtually impossible to realize in the portable telephone the merit of that is that it is small in size and light in weight.
Further, on account of the property of the portable telephone, the environment in that it is used drastically fluctuates. Therefore, it is prerequisite to make the distortion compensation follow such a drastic fluctuation, i.e. an adaptive one. This conversion to an adaptive distortion-compensation has been becoming a very important task along with the conversion of the portable telephone to a small-sized version.
As the conventional techniques, there are several reports including the technique in which pre-distortion is made adaptive, the technique in which feedforward is made adaptive, etc. In the present invention, it is intended to provide an adaptive distortion-compensation apparatus that is used for achieving pre-distortion. Therefore, several conventional examples in that field are illustrated below.
As a first conventional example, there is, for example, 1992. European Microwave Conference. Vol. 22, pp. 1125-pp. 1130, xe2x80x9cPower Amplifier Adaptive Linearization Using Predistortion with Polynominalxe2x80x9d. In FIG. 1, there is illustrated a block diagram of an adaptive distortion-compensation apparatus that is introduced there.
In FIG. 1, assuming that Vout=A (Vin) represents the non-linear input/output characteristic of a power amplifier PA the distortion of that is to be compensated, input base band signals I and Q are arithmetic-operated using a function circuit H (I, Q) for linearizing that A (Vin). And the operated results Ixe2x80x2 and Qxe2x80x2 are made analog by the use of a digital/analog converter D/A and simultaneously are each converted to a high-frequency band. And the resulting signals are input to the power amplifier PA. The resulting amplified outputs Vout are detected and they are converted to base band signals by the use of a demodulator DEM. Signals If and Qf are thereby obtained.
Here, adaptive compensation is performed in such a way as to compare the input signals I and Q with the detection signals If and Qf and to adjust the constants contained in the linearizing function H (I, Q) so that that difference may become zero. This operation is repeatedly carried out until that difference becomes zero. The constants contained in the function H (I, Q) are thereby determined to a final, optimum value.
As a second conventional example, there is, for example, IEEE Transaction on Vechicular Technologies, Vol. 43, No. May 2, 1994, pp. 323-pp. 332. xe2x80x9cAdaptive Linearization Using Pre-distortionxe2x80x9d. In FIG. 2 there is illustrated a block diagram of an adaptive distortion-compensation apparatus that is introduced there.
In FIG. 2, for better understanding of the explanation, the signals that have the same functions as those of the signals of FIG. 1 are represented by the same reference symbols. Generating adresses corresponding ro the input signals I and Q data signal conversion is performed by accessing a conversion table Tbl. Thereby, there are obtained data signals Ixe2x80x2 and Qxe2x80x2 capable of linearizing the power amplifier PA, the data signals Ixe2x80x2 and Qxe2x80x2 being input to the power amplifier PA. It is thereby arranged to detect the amplified output signal Vout and to convert it to a base band signal with use of the demodulator DEM, thereby obtaining the signals If and Qf.
Here, in the adaptive compensation, the input signals I and Q and the detection signals If and Qf are compared with each other. Namely, in a subtractor-SUB, a difference (en) is determined between each of the input signals I, Q and a corresponding one of the detection signals If, Qf. And this conventional technique has an address generation part Adrs. This address generation part is intended to adjust the address in correspondence with that access is had to the table Tbl, so as for that difference (en) to become zero. And, until that difference (en) becomes exactly zero, there is repeatedly adjusted the address generation part Adrs, to thereby optimize the address value for accessing the conversion table Tbl.
In the above-described first and second conventional examples, the constants contained in the linearizing function, or the address for accessing the linearizing table, is optimized. However, in any one of those conventional examples, it is necessary to use a demodulator for converting the output of the power amplifier to a base band signal. Generally, an orthogonal demodulator is used as that demodulator. Therefore, each of those conventional examples has a drawback that the scale of the relevant circuitry becomes greatly massive.
Therefore, the present invention is an adaptive distortion-compensation apparatus that can convert the output of the power amplifier to a base band signal without using an orthogonal demodulator.
An adaptive distortion-compensation apparatus according to the present invention is characterized by comprising: first envelope detection-means for obtaining a first envelope detection signal of an input signal; first storage means, supplied with an address signal based on the first envelope detection signal, for outputting an amplitude compensation data signal corresponding to the address signal; second storage means, supplied with the address signal based on the first envelope detection signal, for outputting an output data signal corresponding to the address signal, and for writing write-in data; latch means for latching output data read out from the second storage means; a power amplifier; second envelope detection means, supplied with an output signal of the power amplifier, for obtaining a second envelope detection signal; signal detection/logical conversion means for outputting plus and minus digital signals in accordance with plus and minus signs of the difference between the first envelope detection signal and the second envelope detection signal; first addition means for adding an output signal of the latch means and an output signal of the sign detection and logical conversion means, and for supplying the added signals to the second storage means as the write-in data; second addition means for adding an output signal of the first storage means and the output signal of the latch means, and for obtaining a second addition output signal; first digital to analog conversion means for converting an output signal of the second addition means into an analog signal, and for supplying the analog signal to the power amplifier; and gain control means, supplied with the input signal, for controlling a gain of the input signal in accordance with an output of the first digital-to-analog conversion means, and for supplying the gain-controlled input signal to the amplification circuit. Further, the adaptive distortion-compensation apparatus comprises third storage means, supplied with the address signal based on the first envelope detection signal, for outputting a phase compensation data signal corresponding to the address signal; second digital to analog conversion means for converting the phase compensating data outputted from the third storage means into an analog signal; and phase control means, supplied with the input signal, for controlling a phase of the input signal in accordance with an output signal of the second digital to analog conversion means, and for supplying the phase-controlled resultant input signal to the gain control means.